The present invention relates in general to a digital signal processing automotive radio system having an analog audio source, and more specifically, to generating an analog composite signal from a stereo signal source for digitizing with a single analog-to-digital (A/D) converter.
Digital signal processing (DSP) is being adopted in many electronics applications, such as digital audio systems, signal measurement and analysis, and communications. DSP uses specialized microprocessors that are adapted to perform the signal processing function digitally and at a high rate of speed. DSP achieves advantages in programming flexibility, inherent stability, increased accuracy, high reliability, and lower costs. In a digital audio system, DSP techniques are employed to control audio functions, such as tone control, concert hall emulation, equalization, demodulation, and noise reduction, for example.
A digital audio system typically employs both analog and digital input signals, such as analog signals from a radio tuner and a cassette tape player and digital signals from a compact disc player. The analog input signals must be converted to digital prior to digital signal processing. Once processed, the audio signals are typically converted to analog signals in a digital-to-analog (D/A) converter prior to amplification by an analog power amplifier and reproduction by analog speakers.
A/D conversion can be achieved using stand-alone integrated circuits. Each IC has an associated part cost and requires its own signal path into the DSP IC, supporting functions (such as a clock), and separate packaging space. Thus, it is desirable to minimize the number of converters to be used. DSP ICs are also produced with some A/D converters already included on the integrated circuit. Since the number of such converters is limited, however, it is necessary to utilize them as efficiently as possible.